The present invention relates generally to a system for canceling interference in a receiver, and more specifically, to a system for canceling internal interference in a time-division multiple access (TDMA) receiver, such as in a GSM receiver (Global System For Mobile Receiver).
GSM receivers typically derive many different clock frequencies from a single reference frequency, and are susceptible to "self-jamming" or interference at particular frequency channels. For example, it is common to use crystal oscillator as a system clock operating at a frequency of either 13 MHz or 19.5 MHz because use of such frequencies permits other clock frequencies to be easily derived. A problem arises when a harmonic of the system clock is the same frequency as one of the GSM communication channels. For example, the 72.sup.nd harmonic of a 13 MHz system clock has energy at 936 MHz (72*13=936). Channel 5 of the GSM system happens to be centered at 936 MHz. Thus, the harmonics produced by the system clock causes interference with GSM channel No. 5. Further, additional interference is caused by other harmonics, which may not necessarily be generated by the system clock, but rather, by other clocks in the system and by spurious mixes of local oscillators. In heterodyne or super-heterodyne receivers, local oscillators, mixers, and various digital clocks emit a wide range of harmonics that interfere with various GSM channels.
One approach to solve this problem in prior art systems is to add shielding to the "noisy" components of the system so that the harmonic energy does not substantially escape or enter the receiver circuitry. This involves using various metal enclosures and screens, and may include specialized printed circuit board design. This, however, adds significant weight and increases the size of the device. Because weight and size are extremely important considerations in the design of a GSM system, such as a cellular telephone, the disadvantages of this approach are apparent. Further, use of shielding increases the cost of the device, which is usually sold in a cost-competitive market.